Field of the Invention
The present invention relates to an antenna device and, more particularly, to an antenna device suitable for NFC (Near Field Communication).
Description of Related Art
In recent years, a mobile electronic device such as a smartphone is equipped with an RFID (Radio Frequency Identification: individual identification by radio waves) system and further equipped with, as a communication means of the RFID, an antenna for performing near field communication with a reader/writer and the like.
Further, the mobile electronic device is provided with a metallic shield so as to protect a built-in circuit from external noise and to prevent unnecessary radiation of noise generated inside the device. Particularly, recently, a housing itself of the mobile electronic device is made of metal instead of resin, considering thinness, light weight, durability against drop impact, design, and the like. Cases where the metallic housing doubles as the metallic shield have been increasing. However, since generally the metallic shield shields electric waves, when an antenna needs to be provided, it is necessary to arrange the antenna at a position not overlapping with the metallic shield. When the metallic shield is arranged over a wide range, arrangement of the antenna becomes a serious problem.
To solve the above problem, in antenna devices disclosed in, e.g., Japanese Patent No. 4,687,832, Japanese Patent Application Laid-Open No. 2002-111363, and Japanese Patent Application Laid-Open No. 2013-162195, an opening is formed in a conductive layer, a slit connecting the opening and an outer edge is formed, and an antenna coil is arranged such that an inner diameter portion thereof overlaps with the opening. In this configuration, current flows in the conductive layer so as to shield a magnetic field generated by flowing of current in a coil conductor, and the current flowing around the opening of the conductive layer passes around the slit, with the result that current flows also around the conductive layer by edge effect. As a result, a magnetic field is generated also from the conductive layer, and the conductive layer makes a large loop of a magnetic flux, thereby increasing a communication distance between the antenna device and an antenna of an apparatus at a communication partner side. That is, it is possible to allow the conductive layer to function as an accelerator for increasing a communication distance of the antenna coil.
However, in the above conventional antenna device, it is necessary to form the opening and slit in the conductive layer, which imposes restrictions on freedom of layout of the antenna coil. For example, if the opening cannot be formed at a desired position due to design restrictions, or if formation of the slit is not allowed even though the opening can be formed, the antenna device cannot be constructed. The same problem occurs when an opening for exposing a lens of a camera module cannot be used as the opening for the antenna coil.